1. Field of the Invention
The present invention relates to polyesters and to their method of preparation.
2. Brief Description of the Prior Art
Low molecular weight polyesters formed from reacting polyols with dicarboxylic acids or anhydrides and monoepoxides are known in the art. Such polyesters are useful as resinous binders in the formulation of high solids coating compositions. Typical of the prior art in the area is U.S. Pat. Nos. 3,928,420; 4,104,240 and 4,113,702. There are problems associated with the polyesters of the above-mentioned prior art. If the choice of reactants and the reaction conditions are not carefully controlled, there can be a considerable amount of polyesterification and unreacted polyol formed. Unreacted polyol results in volatile organic content in the resultant coating composition which is undesirable because of increasingly stringent governmental emission standards. Another problem is associated with the low viscosity of the polyester. Low viscosity is, of course, desirable for the formation of high solids coating compositions. However, if the coating compositions are spray applied, there will be an overspray which misses the substrate and accumulates in the printing booth. The low viscosity uncured resinous materials do not harden with time and can become very tacky and slippery creating a hazard to the personnel working in the area.
The present invention overcomes the aforementioned problems by providing a polyester which results in substantially dry overspray and low volatile contents. In the present invention, a polyol is reacted with a 1,2-dicarboxylic anhydride under reaction conditions sufficient to form the half-ester with substantially no polyesterification product and unreacted polyol. This product is then reacted with a polyepoxide to form the polyester. Reaction with the polyepoxide results in chain extension and molecular weight build up which is sufficient to overcome the tacky overspray problems mentioned above.